Determination of correct horizontal and vertical permeabilities in a deviated well

ABSTRACT

In one method, the permeabilities are obtained by correcting the geometric factor derived from combining the FRA analysis and buildup analysis. In a second method, the permeabilities are obtained by combining the spherical permeability estimated from buildup analysis and the geometric skin factor obtained from history matching the probe-pressure data. In other methods, horizontal and vertical permeabilities are determined by analysis of pressure drawdown made with a single probe of circular aperture in a deviated borehole at two different walls of the borehole.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claim priority from U.S. Provisional Patent ApplicationSer. No. 60/604,552 filed on 26 Aug. 2004, the contents of which areincorporated herein by reference. This application is also acontinuation-in-part of United States patent application Ser. No.11/014,422 filed on Dec. 16, 2004. The application is also related to anapplication of the same inventors being filed concurrently underattorney docket no. 584-40072CP

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to the field of instruments used to samplefluids contained in the pore spaces of earth formations. Morespecifically, the invention is related to methods of determininghydraulic properties of anisotropic earth formations by interpretingfluid pressure and flow rate measurements made by such instruments.

2. Description of the Related Art

Electric wireline formation testing instruments are used to withdrawsamples of fluids contained within the pore spaces of earth formationsand to make measurements of fluid pressures within the earth formations.Calculations made from these pressure measurements and measurements ofthe withdrawal rate can be used to assist in estimating the total fluidcontent within a particular earth formation.

A typical electric wireline formation testing instrument is described,for example, in U.S. Pat. No. 5,377,755 issued to Michaels et al.Electric wireline formation testing instruments are typically loweredinto a wellbore penetrating the earth formations at one end of anarmored electrical cable. The formation testing instrument usuallycomprises a tubular probe which is extended from the instrument housingand then is impressed onto the wall of the wellbore. The probe isusually sealed on its outside diameter by an elastomeric seal or packingelement to exclude fluids from within the wellbore itself from enteringthe interior of the probe, when fluids are withdrawn from the earthformation through the probe. The probe is selectively placed inhydraulic communication, by means of various valves, with samplingchambers included in the instrument. Hydraulic lines which connect theprobe to the various sample chambers can include connection to a highlyaccurate pressure sensor to measure the fluid pressure within thehydraulic lines. Other sensors in the instrument can make measurementsrelated to the volume of fluid which has entered some of the samplechambers during a test of a particular earth formation. U.S. Pat. No.6,478,096 to Jones et al. discloses a formation pressure tester that ispart of a bottomhole assembly used in drilling and can make measurementswhile drilling (MWD).

Properties of the earth formation which can be determined usingmeasurements made by the wireline formation testing instrument includepermeability of the formation and static reservoir pressure.Permeability is determined by, among other methods, calculating a rateat which a fluid having a known viscosity moves through the pore spaceswithin the formation when a predetermined differential pressure isapplied to the formation. As previously stated, the formation testinginstrument typically includes a sensor to make measurements related tothe volume of fluid entering the sample chamber, and further includes apressure sensor which can be used to determine the fluid pressure in thehydraulic lines connecting the probe to the sample chamber. It isfurther possible to determine the viscosity of the fluid in the earthformation by laboratory analysis of a sample of the fluid which isrecovered from the sample chamber.

The permeability of a reservoir is an important quantity to know as itis one of the important factors determining the rate at whichhydrocarbons can be produced from the reservoir. Historically, two typesof measurements have been used for determination of permeability. In theso-called drawdown method, a probe on a downhole tool in a borehole isset against the formation. A measured volume of fluid is then withdrawnfrom the formation through the probe. The test continues with a buildupperiod during which the pressure is monitored. The pressure measurementsmay continue until equilibrium pressure is reached (at the reservoirpressure). Analysis of the pressure buildup using knowledge of thevolume of withdrawn fluid makes it possible to determine a permeability.Those versed in the art would recognize that the terms “permeability”and “mobility” are commonly used interchangeably. In the presentdocument, these two terms are intended to be equivalent.

In the so-called buildup method, fluid is withdrawn from the reservoirusing a probe and the flow of fluid is terminated. The subsequentbuildup in pressure is measured and from analysis of the pressure, aformation permeability is determined.

U.S. Pat. No. 5,708,204 to Kasap having the same assignee as the presentapplication and the contents of which are fully incorporated herein byreference, teaches the Fluid Rate Analysis (FRA) method in which datafrom a combination of drawdown and buildup measurements are used todetermine a formation permeability.

The methods described above give a single value of permeability. Inreality, the permeability of earth formations is anisotropic. It is notuncommon for horizontal permeabilities to be ten or more times greaterthan the vertical permeability. Knowledge of both horizontal andvertical permeabilities is important for at least two reasons. First,the horizontal permeability is a better indicator of the productivity ofa reservoir than an average permeability determined by the methodsdiscussed above. Secondly, the vertical permeability provides usefulinformation to the production engineer of possible flow rates betweendifferent zones of a reservoir, information that is helpful in thesetting of packers and of perforating casing in a well. It is to benoted that the terms “horizontal” and “vertical” as used in the presentdocument generally refers to directions in which the permeability is amaximum and a minimum respectively. These are commonly, but notnecessarily horizontal and vertical in an earth reference frame.Similarly, the term “horizontal” in connection with a borehole is one inwhich the borehole axis is parallel to a plane defined by the horizontalpermeability.

U.S. Pat. No. 4,890,487 to Dussan et al. teaches a method fordetermining the horizontal and vertical permeabilities of a formationusing measurements made with a single probe. The analysis is based onrepresenting the fluid behavior during drawdown by an equation of theform: $\begin{matrix}{{{P_{f} - P_{i}} = \left( {\frac{Q\quad\mu}{2\quad\pi\quad r_{p}k_{h}}\quad{F\left( {\frac{\pi}{2},\sqrt{1 - {k_{V}/k_{H}}}} \right)}} \right)},} & (1)\end{matrix}$where

-   P_(f) represents pressure of the undisturbed formation;-   P_(i) represents pressure at the end of draw-down period i;-   Q_(i) represents volumetric flow rate during draw-down period i;-   μ represents dynamic viscosity of the formation fluid;-   r_(p) represents the probe aperture radius;-   k_(H) represents horizontal formation permeability;-   k_(V) represents vertical formation permeability; and-   F denotes the complete elliptic integral of the first kind.    In Dussan, at least three sets of measurements are made, such as two    drawdown measurements and one buildup measurement, and results from    these are combined with a table lookup to give an estimate of    vertical and horizontal permeability. The above equation was derived    based on several assumptions: an infinite wellbore, constant    drawdown rate and steady state flow. The steady state flow condition    cannot be satisfied in a low permeability formation, or unless a    long test time is used. A constant drawdown rate is not reachable in    practice because the tool needs time for acceleration and    deceleration. The storage effect also makes it difficult to reach a    constant drawdown rate. The infinite wellbore assumption excludes    the wellbore effect on the non-spherical flow pattern, making their    method not inapplicable to high k_(H)/k_(V) cases. The cases of    k_(H)/k_(V)<1 were not presented in Dussan. The method works only in    a homogeneous formation. However, their method does not have any    procedure to check if the condition of homogeneous formation can be    satisfied for a real probe test. The present invention addresses all    of these limitations.

U.S. Pat. No. 5,265,015 to Auzerais et al. teaches determination ofvertical and horizontal permeabilities using a special type of probewith an elongate cross-section, such as elliptic or rectangular.Measurements are made with two orientations of the probe, one with theaxis of elongation parallel vertical, and one with the axis ofelongation horizontal. The method requires a special tool configuration.To the best of our knowledge, there does not exist such a tool and it isprobably difficult or expensive to build one. The present invention doesnot require a special tool, and such tool is available, for example, theone described in U.S. Pat. No. 6,478,096 to Jones et al.

U.S. Pat. No. 5,703,286 to Proett et al. teaches the determination offormation permeability by matching the pressure drawdown and builduptest data (possibly over many cycles). There is a suggestion that themethod could be modified to deal with anisotropy and explicit equationsare given for the use of multiple probes. However, there is no teachingon how to determine formation anisotropy from measurements made with asingle probe. Based on the one equation given by Proett, it would beimpossible to determine two parameters with measurements from a singleprobe. It would be desirable to have a method of determination ofanisotropic permeabilities using a single probe. The present inventionsatisfies this need.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a method of estimating apermeability of an earth formation. The formation contains a formationfluid. Transient pressure measurements are made with a probe in aborehole in the earth formation. The probe has an aperture that issubstantially circular aperture, or non-elliptical in shape. The probeis in hydraulic communication with the earth formation. First and secondestimates of a permeability are obtained from the pressure measurements,and from the first and second estimates of permeability a horizontalpermeability of the formation, and/or a vertical permeability of theformation are determined. The pressure measurements may involve adrawdown involving withdrawal of fluid from the earth formation andstopping the withdrawal: the first estimate of permeability being basedon measurements after the withdrawal while the second estimate is basedon measurements before and after the withdrawal. The method may furtherinvolve determining a ratio of the horizontal permeability to thevertical permeability and/or determining a spherical permeability. Ageometric factor relating to anisotropy and/or a geometric skin factorrelating to non-spherical flow may be used in the determination. Thegeometric skin factor may be obtained by matching pressure measurementsmade before and after the stopping of withdrawal. The probe may beconveyed into the borehole on a wireline, a drilling tubular or aslickline. The borehole may be substantially vertical: if so, the twopermeability estimates are obtained from measurements made by the probein the same orientation in the borehole. The borehole may be a deviatedborehole. If so, the probe may be positioned with its aperturesubstantially in a vertical plane.

Another embodiment of the present invention is an apparatus forevaluating an earth formation which contains a formation fluid. Theapparatus includes a probe with an aperture that is substantiallycircular or non-elliptical is conveyed in the borehole. A pressuresensing device makes transient pressure measurements using the probe. Aestimates first and second permeabilities from the pressuremeasurements, and determines from the first and second estimates ofpermeability a horizontal permeability of the formation, and/or avertical permeability of the formation. The pressure measurements mayinvolve a drawdown involving withdrawal of fluid from the earthformation and stopping the withdrawal: the first estimate ofpermeability being based on measurements after the withdrawal while thesecond estimate is based on measurements before and after thewithdrawal. The processor may further determine a ratio of thehorizontal permeability to the vertical permeability and/or a sphericalpermeability. The processor may also determine a geometric factorrelating to anisotropy and/or a geometric skin factor relating tonon-spherical flow. The processor may obtain the geometric skin factorby matching pressure measurements made before and after the stopping ofwithdrawal. The apparatus may include a wireline, a drilling tubular ora slickline which convey the probe into the borehole. The borehole maybe substantially vertical: if so, the two permeability estimates areobtained from measurements made by the probe in the same orientation inthe borehole. The borehole may be a deviated borehole. If so, the probemay be positioned with its aperture substantially in a vertical plane.

Another embodiment of the invention is a computer readable medium foruse with an apparatus for evaluating an earth formation which contains aformation fluid. The apparatus includes a probe having a substantiallycircular aperture or non-elliptical conveyed in the borehole. The probeis in hydraulic communication with the earth formation. The apparatusalso includes a pressure sensing device which makes transient pressuremeasurements in the probe. The medium includes instructions which enablea processor to estimate a first permeability and a second permeabilityfrom the pressure measurements, and determine from the first and secondpermeability a horizontal permeability of the formation, and/or avertical vertical permeability of the formation. The medium of may be aROM, an EPROM, an EEPROM, a Flash Memory, and/or an optical disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood with reference to theaccompanying figures in which like numerals refer to like elements andin which:

FIG. 1 (prior art) is an illustration of a wireline conveyed formationtesting instrument positioned within a wellbore;

FIG. 2 (prior art) shows a graph of measured pressure with respect tofluid flow rate in the earth formation;

FIG. 3 shows numerical values of the G_(os) in FRA for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V);

FIG. 4 shows numerical values of the s_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V);

FIG. 5 shows numerical values of the r_(ep) for various values ofr_(p)/r_(w), and anisotropy k_(H)/k_(V);

FIG. 6 is an FRA plot for the simulated probe test with k_(H)/k_(V)=10;

FIG. 7 is a plot of pressure changes and pressure derivatives forbuildup data;

FIG. 8 is a flow chart illustrating one embodiment of the presentinvention for determining horizontal and vertical permeabilities frombuildup and FRA analysis;

FIG. 9 is a comparison of simulated pressure data with an analyticalspherical solution derived using the buildup permeability and anisotropic skin factor;

FIGS. 10 a, 10 b shows use of a probe for two measurements in a nearhorizontal borehole;

FIG. 11 shows K values for various values of r_(p)/r_(w), and anisotropyk_(H)/k_(V);

FIG. 12 is a schematic illustration of a probe in a deviated borehole;

FIG. 13 shows exemplary values of the geometric skin factor G_(0sθ) atdifferent deviation angles of a borehole;

FIG. 14 shows exemplary values of the skin factor s_(pθ) at differentdeviation angles; and

FIG. 15 shows K values for different k_(H)/k_(V) at different deviationangles (φ=90′ or 270°)

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated schematically a section ofa borehole 10 penetrating a portion of the earth formations 11, shown invertical section. Disposed within the borehole 10 by means of a cable orwireline 12 is a sampling and measuring instrument 13. The sampling andmeasuring instrument is comprised of a hydraulic power system 14, afluid sample storage section 15 and a sampling mechanism section 16.Sampling mechanism section 16 includes selectively extensible wellengaging pad member 17, a selectively extensible fluid admittingsampling probe member 18 and bi-directional pumping member 19. Thepumping member 19 could also be located above the sampling probe member18 if desired.

In operation, sampling and measuring instrument 13 is positioned withinborehole 10 by winding or unwinding cable 12 from a hoist 19 aroundwhich cable 12 is spooled. Depth information from depth indicator 20 iscoupled to processor 21. The processor analyzes the measurements made bythe downhole tool. In one embodiment of the invention, some or all ofthe processing may be done with a downhole processor (not shown). Asatellite link 23 may be provided to send the data to a remote locationfor processing.

For any formation testing tool, the flow measurement using a singleprobe is the cheapest and quickest way. The present invention providestwo practical methods to estimate horizontal and vertical permeabilitiesfrom such probe test data. The first method is to combine the results ofthe two analyses, FRA and pressure buildup analysis. The second methodis to combine the results of buildup analysis and pressure historymatching. The probe test can be conducted using Baker Atlas's formationtesting tool used under the service mark RCI™. Some details of theformation testing tool are described in U.S. Pat. No. 5,377,755 issuedto Michaels et al., having the same assignee as the present inventionand the contents of which are incorporated herein by reference.

The method of the present invention uses data from a drawdown test and apressure buildup test made with a single probe. The relationship betweenmeasured pressure and formation flow rate can be observed in the graphin FIG. 2. The pressure and flow rate measurements are shown asindividual points connected by a curve 70. A linear regression analysisof the points on curve 70 can be used to generate a line 72 for whichthe slope can be calculated. The slope of line 72 is related to thefluid mobility.

As discussed in Sheng et al., if the non-spherical flow pattern isdescribed using a geometric skin factor, s_(p), the spherical drawdownsolution may be written $\begin{matrix}{{{p_{i} - {p(t)}} = {{\frac{q\quad\mu}{4\quad\pi\quad k_{s}r_{p}}\left( {1 + s_{p}} \right)} - {\frac{q\quad\mu}{4\quad\pi\quad k_{s}}\sqrt{\frac{\phi\quad\mu\quad c_{t}}{\pi\quad k_{s}}}\quad\frac{1}{\sqrt{t}}}}},} & (2)\end{matrix}$where

-   c_(i) is the total formation compressibility, atm⁻¹;-   k_(s) is the spherical permeability, D;-   p(t) represents the measured pressure in the tool, atm;-   p_(i) is the initial formation pressure, atm;-   q is the volumetric flow rate, cm³/s;-   r_(p) is the true probe radius, cm;-   s_(p) is the geometric skin factor, dimensionless;-   t is the time since the start of drawdown, s;-   μ is the viscosity of fluid, cP; and-   φ is the formation porosity, fraction.    The units of measurement are not relevant except as far as they    pertain to specific numerical values derived later in this document.

The steady-state pressure drop for a single probe in an anisotropicformation was investigated by Dussan and Sharma (1992). On the basisthat most of the pressure drop occurs in the vicinity of the probe andthe probe is very small in relation to the wellbore, they treated thewellbore as being infinite in diameter (r_(w)=∞). Their pressure drop isformulated by $\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},{r_{w} = \infty}} \right)}} = {\frac{q\quad\mu}{2\quad\pi\sqrt{k_{H}k_{V}}\quad{\max\left( {r_{p},{r_{p}/\eta}} \right)}}{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}}},} & (3)\end{matrix}$where η=k_(ν)/k_(h), and F(π/2, e) is the complete elliptical integralof the first kind defined as $\begin{matrix}{{F\left( {\frac{\pi}{2},e} \right)} = {\int_{0}^{1}{\frac{\mathbb{d}v}{\sqrt{\left( {1 - v^{2}} \right)\left( {1 - {e^{2}v^{2}}} \right)}}.}}} & (4)\end{matrix}$Note that F tends to π/2 as e defined as √{square root over (1−η²)}tends to zero in an isotropic case.

Wilkinson and Hammond (1990) extended Dussan and Sharma's work toinclude a correction for the borehole radius by introducing a shapefactor, C_(eff). The shape factor is defined as $\begin{matrix}{{C_{eff}\left( {\eta,r_{p},r_{w}} \right)} = {\frac{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}}{\Delta\quad{p\left( {\eta,r_{p},{r_{w} = \infty}} \right)}}.}} & (5)\end{matrix}$Then the pressure drop is $\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}} = \frac{q\quad\mu\quad{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}\quad C_{eff}}{2\quad\pi\sqrt{k_{H}k_{V}}\quad{\max\left( {r_{p},{r_{p}/\eta}} \right)}}},} & (6) \\{where} & \quad \\{C_{eff} = {1 - {{\frac{\max\left( {r_{p},{r_{p}/\eta}} \right)}{4\quad r_{w}\quad{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}}\left\lbrack {3.3417 + {\ln\left( {\frac{r_{w}\eta}{2\quad{r_{p}\left( {1 + \eta} \right)}} - \frac{1}{1 + \eta}} \right)}} \right\rbrack}.}}} & (7)\end{matrix}$

When the wellbore radius tends to infinity, Ceff tends to 1, and eqn. 6becomes identical to eqn. 3, as should be the case. In the FRAformulation, non-spherical flow geometry is considered by introducing ageometric factor, G₀. The pressure drop induced by a flow rate is$\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}} = \frac{q\quad\mu}{G_{0}k_{FRA}r_{p}}},} & (8)\end{matrix}$where k_(FRA) is the permeability estimated from the FRA technique.

By comparing eqns. 6 and 8, the values of G_(o) can be derived from thevalues of F and C_(eff) fusing the following equation $\begin{matrix}{G_{0} = {\frac{2\quad\pi\quad\sqrt{k_{H}k_{V}}\quad{\max\left( {r_{p},{r_{p}/\eta}} \right)}}{{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}\quad C_{eff}r_{p}k_{FRA}}.}} & (9)\end{matrix}$Deriving the values of G₀ using the above equation depends on whichpermeability k_(FRA) is (horizontal, vertical or sphericalpermeability). It also involves the calculation of the completeelliptical integral. Sometimes such calculation may not be performedeasily, especially when η² is greater than one. It is also found thatthe values of C_(eff) calculated using eqn. 7 are even larger than 1.0in the cases of high k_(H)/k_(V), which violates the fluid flow physics.This is attributable to violation of one of the assumptions used in thederivation of eqn 7 when k_(H)/k_(V) is very large. In other words, eqn.7 is not applicable in some cases. As a result, we may not be able touse eqn. 6 to calculate the pressure drop in some cases.

Wilkinson and Hammond (1990) corrected the values of C_(eff) in thecases of high k_(H)/k_(V). Based on the corrected C_(eff), they definedanother parameter, k_(H)/k_(D). Here k_(H) is horizontal permeability,and k_(D) is a drawdown permeability, defined as $\begin{matrix}{k_{D} = {\frac{q\quad\mu}{4\quad r_{p}\Delta\quad p}.}} & (10)\end{matrix}$k_(D) is computed as if the flow occurs in an isotropic formation andthe borehole is infinite. In this case, the flow is a hemi-sphericalflow, and the equivalent probe radius is 2r_(p)/π. When eqn. 10 is usedin an isotropic formation with an infinite wellbore, the estimated k_(D)is the true formation permeability. When eqn. 10 is used in a realanisotropic formation with a real finite wellbore, the estimated k_(D)may not represent the horizontal, vertical, or spherical permeabilityand is a function of k_(H)/k_(V) and r_(p)/r_(w). Because k_(D) is afunction of k_(H)/k_(V) and r_(p)/r_(w), we can use its values to derivethe values for other geometric correction factors at differentk_(H)/k_(V) and r_(p)/r_(w).

To estimate G_(o), we compare eqn. 8 with eqn. 10, and get$\begin{matrix}{{G_{0}k_{FRA}} = {{4\quad k_{D}} = {\frac{q\quad\mu}{4\quad r_{p}\quad\Delta\quad p}.}}} & (11)\end{matrix}$We note for a particular test that with the measured q and Δp, and thefixed μ, r_(p), the product, G_(o)k_(FRA), is fixed. G_(o) and k_(FRA)are related to each other by the relationship described by the aboveequation. In other words, depending on the type of permeability sought(e.g., horizontal, vertical, or spherical permeability), differentvalues of G_(o) are required. From eqn. 11, $\begin{matrix}{G_{0} = {\frac{4k_{0}}{k_{FRA}}.}} & (12)\end{matrix}$If a spherical permeability from FRA is sought, then it is necessary touse the G_(os) which corresponds to spherical permeability:$\begin{matrix}{G_{os} = {\frac{4}{k_{s}/k_{D}} = {\frac{4}{\left( {k_{H}^{2/3}{k_{V}^{1/3}/k_{D}}} \right)} = {\frac{4}{k_{H}/k_{D}}{\left( \frac{k_{H}}{k_{V}} \right)^{1/3}.}}}}} & (13)\end{matrix}$

Here spherical permeability has been assumed to be given byk_(s)=√{square root over (k_(V)/k_(H) ²)}. From the published values ofk_(H)/k_(D) (Wilkinson and Hammond, 1990), the values of G_(os) arereadily obtained. The values as a function of r_(p)/r_(w) andk_(H)/k_(V) are tabulated in Table 1 and shown in FIG. 3. TABLE 1Numerical values of G_(os) in FRA for various values of r_(p)/r_(w) andanisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.025 0.05 0.1 0.2 0.30.01 3.75 3.75 3.75 3.92 3.92 0.1 3.64 3.64 3.71 3.87 3.95 1 4.08 4.174.26 4.44 4.65 10 5.42 5.56 5.78 6.11 6.38 100 8.33 8.60 9.06 9.72 10.261000 14.18 14.87 15.81 17.09 18.02 10000 25.96 27.45 29.31 31.57 33.15100000 49.64 52.60 55.92 59.89 62.51 1000000 97.09 102.30 108.40 115.27119.76From Table 1 and FIG. 3, we see that the geometric factor is a strongfunction of anisotropy and a weak function of r_(p)/r_(w). Also, thevalues of G_(os) for k_(H)/k_(V) from 1 to 100 calculated from eqn. 13are in close agreement with those calculated from eqn. 9 in whichC_(eff) is calculated using eqn. 7.

The concept of geometric skin was proposed to represent the abovedefined geometric factor (Strauss, 2002). Defining a geometric skinfactor s_(p) to account for the deviation from the true spherical flowgives $\begin{matrix}{{\Delta\quad p} = {\frac{q\quad{\mu\left( {1 + s_{p}} \right)}}{4\quad\pi\quad k_{s}r_{p}}.}} & (14)\end{matrix}$Comparing eqns. 10 and eqn. 14, s_(p) can be estimated from$\begin{matrix}{s_{p} = {\frac{\pi\left( {k_{H}/k_{V}} \right)}{\left( {k_{H}/k_{V}} \right)^{1/3}} - 1.}} & (15)\end{matrix}$

Again from the published values of k_(H)/k_(D) (Wilkinson and Hammond,1990), the values of s_(p) are readily obtained. The values as afunction of r_(p)/r_(w) and k_(H)/k_(D) are tabulated in Table 2 andshown in FIG. 4. TABLE 2 Numerical values of s_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 2.35 2.35 2.35 2.21 2.21 0.1 2.45 2.45 2.38 2.252.18 1 2.08 2.02 1.95 1.83 1.70 10 1.32 1.26 1.17 1.06 0.97 100 0.510.46 0.39 0.29 0.23 1000 −0.11 −0.15 −0.21 −0.26 −0.30 10000 −0.52 −0.54−0.57 −0.60 −0.62 100000 −0.75 −0.76 −0.78 −0.79 −0.80 1000000 −0.87−0.88 −0.88 −0.89 −0.90Again, there is little dependence on the probe packer size as measuredby the dimensionless probe size, r_(p)/r_(w).

If we define an equivalent probe radius, r_(ep), to account for thedeviation from true spherical flow, we can write $\begin{matrix}{{\Delta\quad p} = {\frac{q\quad\mu}{4\quad\pi\quad k_{s}r_{sp}}.}} & (16)\end{matrix}$Comparing eqns. 10 and 16, r_(ep) can be estimated from $\begin{matrix}{r_{ep} = {\frac{{r_{p}\left( {k_{H}/k_{V}} \right)}^{1/3}}{\pi\left( {k_{H}/k_{D}} \right)}.}} & (17)\end{matrix}$

Using the published values of k_(H)/k_(D) (Wilkinson and Hammond, 1990),the values of r_(ep) are readily obtained. The values as a function ofr_(p)/r_(w) and k_(H)/k_(V) are tabulated in Table 3 and shown in FIG.5. TABLE 3 Numerical values of the r_(ep)/r_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 0.30 0.30 0.30 0.31 0.31 0.1 0.29 0.29 0.30 0.310.31 1 0.32 0.33 0.34 0.35 0.37 10 0.43 0.44 0.46 0.49 0.51 100 0.660.68 0.72 0.77 0.82 1000 1.13 1.18 1.26 1.36 1.43 10000 2.07 2.18 2.332.51 2.64 100000 3.95 4.19 4.45 4.77 4.97 1000000 7.73 8.14 8.63 9.179.53

The formulations and values of the above correction factors are based onthe related spherical flow eqns. 8, 14, or 16. For eqn. 6, k_(FRA) isassumed to be the spherical permeability. Comparing their defining eqns.11, 13, and 15, it can be seen that these three correction factors havethe following relationship: $\begin{matrix}{{{{s_{p} + 1} = {\frac{4\pi}{G_{os}} = \frac{1}{r_{ep}/r_{p}}}};}{or}} & (18) \\{{{s_{p} + 1} = \frac{4\pi}{G_{os}}},} & (19) \\{\frac{1}{r_{ep}/r_{p}} = {\frac{4\pi}{G_{os}}.}} & \left( {19a} \right)\end{matrix}$Substituting from eqn. 19 into eqn. 2, gives: $\begin{matrix}{{p_{i} - {p(t)}} = {\frac{q\quad\mu}{G_{os}k_{s}r_{p}} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{{\phi\mu}\quad c_{t}}{\pi\quad k_{s}}}{\frac{1}{\sqrt{t}}.}}}} & (20)\end{matrix}$Eqns. 2 and 20 are valid for both isotropic and anisotropic formations.Using the principle of superposition, the buildup solution is$\begin{matrix}{{{p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{{\phi\mu}\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}},} & (21)\end{matrix}$where Δt is the shut-in time, s. Here, q is the flow rate for theprevious drawdown measurement. According to eqn. 20, the buildupmobility is estimated from $\begin{matrix}{{\left( \frac{k_{s}}{\mu} \right)_{BU} = {\frac{1}{\pi}\left( \frac{q}{4\quad m_{s}} \right)^{2/3}\left( {\phi\quad c_{t}} \right)^{1/3}}},} & (22)\end{matrix}$where m_(s) is the slope of the linear plot of p(t) vs. the timefunction (Δt^(−1/2)−t^(−1/2)). For the purposes of the presentinvention, the permeability measured using the buildup measurements isreferred to as a first permeability.

Turning now to the FRA method as described in Kasap, $\begin{matrix}{{{p(t)} = {p_{i} - \frac{q_{f}\mu}{k_{s}G_{os}r_{p}}}},} & (23)\end{matrix}$where q_(f), the formation flow rate at the sand face near the probe, is$\begin{matrix}{{q_{f} = {{c_{sys}V_{sys}\frac{{dp}(t)}{dt}} + q_{dd}}},} & (24)\end{matrix}$corrected for the storage effect. In the above equation, c_(sys) is thecompressibility of the fluid in the tool, atm⁻¹; q_(dd) is the pistonwithdrawal rate, cm³/s; V_(sys) is the system (flow line) volume, cm³.

According to FRA, the data in both drawdown and buildup periods arecombined to estimate the mobility from $\begin{matrix}{{\left( \frac{k_{s}}{\mu} \right)_{FRA} = \frac{1}{G_{os}r_{p}m_{FRA}}},} & (25)\end{matrix}$where m_(FRA) is the slope of the linear plot of p(t) vs. q_(f). Byplotting the drawdown data and the buildup data in the FRA plot (FIG.2), if both data are seen to fall on the same straight line with aslope, m_(FRA), the estimated permeability from the drawdown and thebuildup is the same. That means within the radius of investigation forthe drawdown and buildup, the formation is homogeneous. This is thecondition for the presented methods to work.

Eqn. 25 shows that the estimated mobility from FRA is affected by thelocal flow geometry indicated by G_(os). Thus, a correct value of G_(os)must be provided. However, G_(os) strongly depends on the ratio ofvertical-to-horizontal permeability that is generally unknown before thetest is performed. In this case, the value of G_(os) in an isotropicformation is used. As a result, the FRA estimated permeability may notrepresent the true spherical permeability. In contrast, the sphericalpermeability can be obtained from a buildup analysis without priorknowledge of formation anisotropy, and the estimate of mobility from thebuildup analysis is not affected by the local flow geometry according toeqn. 22. In other words, the correct estimate of spherical permeabilitycan be obtained from buildup analysis without knowing formationanisotropy and local flow geometry. The difference in the estimatedspherical permeability from buildup analysis and FRA, discussed in theabove, can be used to estimate the horizontal and verticalpermeabilities. For the purposes of the present invention, thepermeability determined by FRA processing is referred to as a secondpermeability.

The difference in the estimated spherical permeability from buildupanalysis (the first permeability) and FRA permeability (the secondpermeability), discussed in the above, can be used to estimate thehorizontal and vertical permeabilities. A simulated probe-pressure testdata as an example is used to illustrate the procedures. First, theprobe-pressure test simulation is described.

The simulation model used is given in Table 4. TABLE 4 Input parametersused in simulation Porosity, fraction 0.2 Spherical permeability, mD 10k_(H)/k_(V) 10 Viscosity, cP 1 Formation pressure, psi 4000 Fluidcompressibility, 1/psi 2.50E−06 Wellbore radius, cm. 6.35 Probe radius,cm 0.635 Flow line volume, ml 371 Drawdown rate, ml/s 4 Duration ofdrawdown, s 10The symmetry in the problem is used to reduce the model to one quarterof the probe and the formation. Further, the effect of gravity isneglected. The model is a radial model. The k_(H)/k_(V) is equal to 10with the spherical permeability of 10 mD. The r_(p)/r_(w), is equal to0.1. The drawdown rate for the quarter model is 1 ml/s.

Next, results of analyzing the simulation data using the FRA techniqueare discussed. FIG. 6 shows the expected linear relation between thepressure and the formation flow rate. If the data were realprobe-pressure test data and k_(H)/k_(V) were unknown, one couldlogically assume the formation were isotropic. According to Table 1, thegeometric factor, G_(os) would be 4.26 for r_(p)/r_(w), equal to 0.1.Based on eqn. 25 and using this value of G_(os) one would estimate aspherical permeability of 13 mD.

The simulated pressure test data could also be analyzed using buildup(BU) analysis using any pressure transient analysis software withspherical flow solutions. For this example, the commercially availablesoftware Interpret2003 of Paradigm Geophysical Co was used. FIG. 7 showsthe buildup analysis plot to estimate the spherical permeability forthis case. The abscissa is time and ordinate is the pressure change 701or the pressure derivative 703. The plot is on a log-log scale. Alsoshown on the plot are lines with a slop of +1 (705) and a slope of −½(707). The spherical flow regime is identified by a negative half slopein the log-log derivative plot. From this buildup analysis, thespherical permeability is estimated to be 9.62 mD, close to the inputspherical permeability. It should be noted that the use of theInterpret2003 software is for exemplary purposes only and other softwarepackages that perform similar functions (as described below) could beused.

For the same pressure data, different estimates of permeability areobtained from buildup analysis and from FRA. One is 13 mD from FRA, theother is 9.62 mD from the BU analysis. The latter is close to the actualpermeability used in the simulation model. The former is different fromthe actual permeability because we used an incorrect G_(os). To make FRAestimated permeability closer to the actual one used in the simulation,a value of G_(os) appropriate for the permeability anisotropy ratio inthe simulation should be used. Assuming the BU estimated sphericalpermeability is correct, the correct G_(os) can be estimated as follows.$\begin{matrix}{{\left( {k_{s}G_{os}} \right) = \frac{\mu}{r_{p}m_{FRA}}},} & (26)\end{matrix}$The above shows that for a particular test, since the linearrelationship between the measured q and Δp results in a constant slope,m_(FRA), for the fixed μ and r_(p), the product, (G_(os)k_(s))_(FRA), isfixed. In other words, for a particular test, if an isotropic formationis assumed for FRA, then (G_(os)k_(s)) in the isotropic formation,denoted by (G_(os)k_(s))_(iso), should equal the permeability-geometricfactor product of the anisotropic formation, (G_(os)k_(s))_(ani). Thisproduct consists of the correct G_(os) and the correct k_(s) in theanisotropic formation. Because the BU estimated permeability is assumedto be the true spherical permeability, then the correct G_(os) in theanisotropic formation, (G_(o))_(ani), can be estimated from$\begin{matrix}{\left( G_{os} \right)_{ani} = {\frac{\left( {G_{os}k_{s}} \right)_{iso}}{\left( k_{s} \right)_{BU}}.}} & (27)\end{matrix}$

In the term (G_(os)k_(s))_(iso) of the above equation, G_(os) is thegeometric factor for an isotropic formation (G_(os)=4.26 from Table 1),and k_(s) is the FRA permeability estimated initially assuming theformation is isotropic. For this example, k_(s) is 13 mD. In thedenominator, (k_(s))_(BU) is the spherical permeability estimated fromthe buildup analysis which is 9.62 mD in this example. Therefore, thecorrect G_(os) in this example is $\begin{matrix}{\left( G_{os} \right)_{ani} = {\frac{\left( {G_{os}k_{s}} \right)_{iso}}{\left( k_{s} \right)_{BU}} = {\frac{(4.26)(13)}{9.62} = {5.76.}}}} & (28)\end{matrix}$The estimated G_(os) of 5.76 is very close to the G_(os) in FIG. 1 whenk_(H)/k_(V) is equal to 10 and r_(p)/r_(w) is equal to 0.1. Therefore,by combining the results of FRA and buildup analysis, it is possible todetermine k_(H)/k_(V). Having k_(H)/k_(V) determined, the horizontalpermeability and vertical permeability are readily obtained:k _(H)=(k _(S))_(Bu)/(k _(H) /k _(V))^((1/3)),  (29),k _(V) =k _(H)/(k _(H) /k _(V)).  (30).

For this example, the calculated horizontal permeability and verticalpermeability are 20.7 mD and 2.07 mD, respectively. These values arevery close to their respective simulation model input values of 21.54 mDand 2.15 mD. Thus the method to combine FRA and buildup analysis isdemonstrated.

FIG. 8 is a flow chart illustrating the first embodiment of theinvention. Pressure buildup data 751 are analyzed to get a firstestimate of spherical permeability. Separately, the pressure buildupdata 751 and the drawdown data 757 are analyzed to get a second estimateof spherical permeability 759. Using the two different permeabilities,the geometric factor G_(os) for the probe is corrected 755 and using thecorrected G_(os), the horizontal and vertical permeabilities aredetermined as discussed above.

A second embodiment of the present invention uses the sphericalpermeability obtained from the pressure buildup test (the firstpermeability) as a starting point for matching the entire pressurehistory, including the drawdown data. In Interpret2003 the geometricskin factor, s_(p), is used to describe the non-spherical flow near theprobe. Even though the local geometry near the probe does not affect thepermeability estimate, it does affect the pressure data as given by eqn.2. In the above example, using the BU estimated permeability of 9.62 mDand an isotropic geometric skin factor of 1.95 shown in Table 1, thepressure data from Interpret2003 cannot be matched with the simulatedpressure data because we used the wrong isotropic geometric skin factor.This is shown in FIG. 9 where the abscissa is time and the ordinate ispressure. The buildup portion is used to derive the permeability andthis derived permeability is used to model the pressure data. Moreobviously, the modeled drawdown data 803 does not match the actualdrawdown data 801. To match the simulated pressure data, it is necessaryto use the BU estimated spherical permeability, and also to change thevalue of sp until the pressure data from Interpret2003 matches thenumerical simulation data. It is found that using a value of s_(p) equalto 1.2, a good match is obtained (not shown). From Table 2 it can beseen that s_(p) equal to 1.2 (close the sp of 1.17 in Table 1)corresponds to k_(H)/k_(V) equal to 10 and r_(p)/r_(w) equal to 0.1. Asabove, k_(H)/k_(V) has been estimated to be equal to 10. Oncek_(H)/k_(V) is obtained, eqns 28 and 29 can be used to estimate thehorizontal and vertical permeabilities. Thus, the second method alsouses a permeability from BU analysis (the first method) in combinationwith matching the entire pressure data (processing of data over theentire time interval including drawdown and buildup) to estimatehorizontal and vertical permeabilities.

Conceptually, the second method is based on deriving a sphericalpermeability based on a buildup analysis, and then using this determinedspherical permeability to match the pressure history data by adjustingthe geometric skin factor. Knowledge of the spherical permeability andthe geometric skin factor makes it possible to determine the horizontaland vertical permeabilities.

The two embodiments of the present invention discussed above are used toestimate horizontal and vertical permeabilities based on the assumptionof a homogeneous and anisotropic formation. Such an assumption isreasonable in a practical probe test, because the formation on the smallscale near the probe probably can be considered virtually homogeneous.Therefore, the invention provides a way to estimate the horizontal andvertical permeabilities from a single probe test without additionalinformation. This is in contrast to prior art methods that requiresimultaneous measurements with multiple probes, or measurements with aspecially designed probe in two orientations.

In another embodiment of the invention, two tests are made in a nearhorizontal borehole. In one test, the probe is set and sealedhorizontally against a side wall of the borehole. This is schematicallyillustrated in FIG. 10 a wherein the borehole 851 is shown incross-section and a probe 853 is in contact with the side wall of theborehole. In a second test, schematically illustrated in FIG. 10 b, theprobe 853 is shown against the upper wall of the borehole. It is to benoted that the method is equally applicable if, in the second test, theprobe is against the bottom wall of the borehole.

The solution for the first test is the same as that in a vertical well,and has been discussed above. The solution for the second test isderived next. The objective is to determine the relationship between thepressure at the probe and the fluid withdrawal rate from the anisotropicformation. As before, a cylindrical coordinate system is used in whichthe wellbore wall near the probe can be approximated by the z=0 plane,with the formation located in the half-space z≧0. The initial formationpressure is p_(i). The z axis for the test of FIG. 10 b coincides withthe vertical direction. The perimeter of the probe opening through whichfluid flows is given by r²=r_(p) ² at z=0. The flowing pressure at theprobe opening is pp. There is no flow across the rest of the plane atz=0. The mathematical description of such probe test is a mixed boundaryproblem. Its formulation is given as follows. $\begin{matrix}{{{{k_{H}\left( {\frac{\partial^{2}p}{\partial r^{2}} + {\frac{1}{r}\frac{\partial p}{\partial r}}} \right)} + {k_{V}\frac{\partial^{2}p}{\partial z^{2}}}} = 0},} & (31) \\{{p = {{{p_{p}\quad{at}\quad r} \leq {r_{p}\quad{and}\quad z}} = 0}},} & (32) \\{{\frac{\partial p}{\partial z} = {{{0\quad{at}\quad r} > {r_{p}\quad{and}\quad z}} = 0}},} & (33) \\{\left. p\rightarrow\left. {{p_{i}\quad{as}\quad r^{2}} + z^{2}}\rightarrow{{\infty\quad{and}\quad z} \geq 0} \right. \right.,} & (34)\end{matrix}$

Of interest is the relationship between pressure drop, p_(i)-p_(p), andflow rate, q. This is done by evaluating the integral: $\begin{matrix}{q = {\frac{2{\pi k}_{V}}{\mu}{\int_{A_{p}}{\frac{\partial p}{\partial z}{_{z = 0}{r\quad{{\mathbb{d}r}.}}}}}}} & (35)\end{matrix}$In the above equations,

A_(p) represents area of probe opening, cm²

-   k_(H) represents horizontal permeability, D-   k_(V) represents vertical permeability, D-   p represents pressure, atm-   p_(i) represents initial formation pressure, atm-   p_(p) represents pressure at the probe, atm-   q represents volumetric flow rate, cm³/s-   r represents radial coordinate of cylindrical grid system, cm-   r_(p) represents true probe radius, cm-   z represents z axis in the coordinate system, cm-   μ represents viscosity of fluid, cP    The units of measurement are not relevant except as far as they are    consistently follow one unit system. Here Darcy unit system is used.

Using the following notation: $\begin{matrix}{{r^{\prime} = r},} & {(36),} \\{{z^{\prime} = {\sqrt{\frac{k_{H}}{k_{V}}}z}},} & {(37),}\end{matrix}$the above mathematical formulation (Eqns 31 to 35) is converted in thefollowing formulation: $\begin{matrix}{{{\left( {\frac{\partial^{2}p}{\partial\quad r^{\prime 2}} + {\frac{1}{r^{\prime}}\frac{\partial p}{\partial r^{\prime}}}} \right) + \frac{\partial^{2}p}{\partial\quad z^{\prime 2}}} = 0},} & {\left( 31^{\prime} \right),} \\{{p = {{{p_{p}\quad{at}\quad r^{\prime}} \leq {r_{p}\quad{and}\quad z^{\prime}}} = 0}},} & {\left( 32^{\prime} \right),} \\{{\frac{\partial p}{\partial z^{\prime}} = {{{0\quad{at}\quad r^{\prime}} > {r_{p}\quad{and}\quad z^{\prime}}} = 0}},} & {\left( 33^{\prime} \right),} \\{\left. p\rightarrow\left. {{p_{i}\quad{as}\quad r^{\prime 2}} + \frac{z^{\prime 2}}{k_{H}/k_{V}}}\rightarrow{{\infty\quad{and}\quad z^{\prime}} \geq 0} \right. \right.,} & {\left( 34^{\prime} \right),} \\{q = {\frac{2\pi\sqrt{k_{H}k_{V}}}{\mu}{\int_{A_{p}}{\frac{\partial p}{\partial z^{\prime}}{_{z^{\prime} = 0}\quad{r^{\prime}{{\mathbb{d}r^{\prime}}.}}}}}}} & {\left( 35^{\prime} \right).}\end{matrix}$

The solution for the above problem was solved by Carslaw, H. S. andJaeger, J. C., Conduction of Heat in Solids, Oxford University Press(1959). According to their solution, the relationship between pressuredrop and flow rate for the above problem is $\begin{matrix}{q = {\frac{4\sqrt{k_{H}k_{V}}{r_{p}\left( {p_{i} - p_{p}} \right)}}{\mu}.}} & {(38).}\end{matrix}$Note that from the above equation, it is possible to obtain apermeability (k_(H)k_(V))^(1/2), a geometric average permeability ofhorizontal permeability and vertical permeability.

For the first test with the probe set horizontally against the side wall(FIG. 10 a) in a horizontal well, the relationship between the pressuredrop and flow rate is the same as that in a vertical well. Using thegeometric factor and horizontal permeability, the relationship derivedabove is $\begin{matrix}{{{p_{i} - p_{p}} = \frac{q\quad\mu}{G_{oH}k_{H}r_{p}}},} & {(39),}\end{matrix}$

where G_(oH) is the geometric factor when the pressure drop vs. flowrate relationship is formulated using horizontal permeability, k_(H).Its values at different k_(H)/k_(V) and r_(p)/r_(w) are reported in thesame reference and reprinted here in Table 5. Here r_(w) is the radiusof wellbore. Note that the values in Table 5 are for G_(0H), related toa horizontal permeability whereas the values in Table 1 are for G_(0S),related to a spherical permeability. TABLE 5 Numerical values ofG_(oH)(for k_(H)) for various values of r_(p)/r_(w) and anisotropyk_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.025 0.05 0.1 0.2 0.3 0.01 17.3917.39 17.39 18.18 18.18 0.1 7.84 7.84 8.00 8.33 8.51 1 4.08 4.17 4.264.44 4.65 10 2.52 2.58 2.68 2.84 2.96 100 1.79 1.85 1.95 2.09 2.21 10001.42 1.49 1.58 1.71 1.80 10000 1.20 1.27 1.36 1.47 1.54 100000 1.07 1.131.20 1.29 1.35 1000000 0.97 1.02 1.08 1.15 1.20

From Eqn. 39, the horizontal permeability can be obtained. But thispermeability is closely related to the geometric factor which is astrong function of k_(H)/k_(V). Before analyzing the test data,k_(H)/k_(V) is unknown. However, for a particular test with the measuredq and p_(p), and the fixed μ, r_(p), the product G_(oH)k_(H) is adetermined quantity. For the second test in a horizontal well when theprobe is set vertically against the top wall of the borehole (FIG. 10b), the relationship between the pressure drop and flow rate isdescribed by Eqn. 38 and a mean permeability, (k_(H)k_(V))^(1/2) can beobtained. In other words, when the two tests are conducted at the samemeasured depth, the following two quantities are obtained:$\begin{matrix}{{{K_{S} \equiv {G_{oH}k_{H}}} = \frac{q_{S}\mu}{r_{p}\left( {p_{i} - p_{p,S}} \right)}},} & {(40),} \\{{{K_{T} \equiv \sqrt{k_{H}k_{V}}} = \frac{q_{T}\mu}{4{r_{p}\left( {p_{i} - p_{p,T}} \right)}}},} & {(41),}\end{matrix}$where the subscripts S and T means the probe is set horizontally againstthe side wall and vertically against the top wall, respectively. BothK_(S) and K_(T) are functions of permeability anisotropy, k_(H)/k_(V).Now we define another quantity K using these two quantities:$\begin{matrix}{{K \equiv \frac{K_{S}}{K_{T}}} = {G_{oH}{\sqrt{\frac{k_{H}}{k_{V}}}.}}} & {(42).}\end{matrix}$

Because G_(oH) function of k_(H)/K_(V), K is also a function ofk_(H)/k_(V). Using the values of G_(oH) in Table 4, the values of K areobtained as shown in Table 6 and FIG. 11 as a function of r_(p)/r_(w)and k_(H)/k_(V). For the two pretests conducted at the same measureddepth, the K value can be calculated using K_(S) and K_(T) from Eqns. 10and 11. Then the k_(H)/k_(V) at the measured depth can be obtained bylooking up Table 6 or FIG. 11 using the calculated K value and the knownvalue of r_(p)/r_(w). From knowledge of k_(H)/k_(V), the horizontal andvertical permeabilities are readily determined: $\begin{matrix}{{k_{H} = {K_{T}\sqrt{\frac{k_{H}}{k_{V}}}}},} & (43) \\{k_{V} = {\frac{k_{H}}{\left( {k_{H}/k_{V}} \right)}.}} & (44)\end{matrix}$ TABLE 6 Numerical values of K for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 1.74 1.74 1.74 1.82 1.82 0.1 2.48 2.48 2.53 2.642.69 1 4.08 4.17 4.26 4.44 4.65 10 7.96 8.16 8.49 8.97 9.37 100 17.9418.52 19.51 20.94 22.10 1000 44.86 47.02 50.00 54.06 56.98 10000 120.48127.39 136.05 146.52 153.85 100000 338.21 358.33 381.00 408.04 425.901000000 970.87 1023.02 1084.01 1152.74 1197.60

The above equations are derived based on the assumptions of a constantwithdrawal rate and steady state flow. In a low permeability formation,the steady state flow condition cannot be satisfied unless a long testtime is used. A constant drawdown rate is not reachable in practicebecause the tool needs time for acceleration and deceleration. Thestorage effect also makes it difficult to reach a constant rate. In analternate embodiment of the present invention, both drawdown and builduptests are made at substantially the same depth with the probe against asidewall and an upper (or lower) wall. The Formation Rate Analysis (FRA)presented in U.S. Pat. No. 5,708,204 to Kasap, the contents of which areincorporated herein by reference, are used to calculate the above K_(S)and K_(T).

The measurements made in a near horizontal borehole are a special caseof the more general situation in which two measurements are made in adeviated borehole with an arbitrary deviation angle. The general case isdiscussed with reference to FIG. 12.

The trajectory of a deviation well can be described by the threeparameters: measured depth, deviation angle θ and the azimuth φ withreference to the positive X direction in the horizontal XY plane, as isshown in FIG. 12, a schematic of well trajectory and probe setting in adeviated well 903. The plane defined by the Z axis and the wellbore axis901 is the YZ plane. The deviation angle θ shown in the figure is theangle between the Z axis and the wellbore axis 901. Here we discuss fourspecial positions around the wellbore to set the probe: Positions 1 to 4as shown by the numbers in FIG. 12.

At Position 1 (φ=0°), the probe axis is perpendicular to the YZ plane,so that the probe opening plane is parallel to the YZ plane. Similarlythe probe opening plane is perpendicular to the X axis. It is a specialvertical plane. Although the well is a deviated well, the probe openingplane at this position is the same as that in a vertical well. AtPosition 2 (φ=90°), the probe opening plane is perpendicular to the YZplane. The probe opening plane at Position 3 (φ=180°) is parallel withand of the same vertical position as that at Position 1. The probeopening plane at Position 4 (φ=270°) is parallel with and below that atposition 2. The flow geometry near the probe at Positions 1 and 3 arethe same, and the flow geometry at Positions 2 and 4 are the same in ahomogeneous and anisotropic formation.

One embodiment of the present invention relates to the determination ofthe correct spherical permeability, horizontal permeability and verticalpermeability by conducting two probe tests in a deviated well using anormal probe with a circular cross-section. The two tests are conductedat the same measured depth. Theoretically, the probe can be set at anypositions around the wellbore. However, the solutions needed foranalysis are convenient at the four special positions as identifiedabove. Therefore, we will describe the cases when the probe is set atthese special positions in this invention. If a probe is set at anarbitrary position, the solution presented in this invention needs to bemodified. It is understood that the modifications of correspondingsolutions and analyses fall within the true spirit and scope of thisinvention. In any case, we need to define the values of geometric factorG_(os) to consider the flow geometry near the probe in a deviated well,as we did in a vertical well.

Since the flow geometry changes at different positions, the geometricfactor values will be different at different positions. In general, thegeometric factor G_(os) is a function of θ, φ, r_(p)/r_(w), andk_(H)/k_(V). As noted above we know the effect of r_(p)/r_(w) is notsignificant. Therefore, for brevity, we assume r_(p)/r_(w) equal to0.025 in presenting this invention. Also as discussed above, we onlydiscuss the geometric factor values at the special positions (φ=0°, 90°, 180° and 270°) The flow geometry at Positions 1 (φ=0°) or 3 (φ=180°)in a deviated well are the same as that in a vertical well. Thegeometric factor values at these positions will be the same as those fora vertical well. The values were presented above. At positions at 2(φ=90°) or 4 (φ=270°), the geometric factor values in a deviated wellhave not been discussed previously.

When the deviation angle is 0°, a deviation well becomes a verticalwell. At Positions 2 or 4, the probe opening plane becomes a verticalplane. The values of geometric factors were presented above. When thedeviation angle is 90°, a deviated well becomes a horizontal well. AtPositions 2 or 4, the probe opening plane becomes a horizontal plane.The geometric factor values for such a plane have been derived above.Since we have already had the geometric factor values for the specialangles 0° and 90° we may simply use a linear interpolation to derive thevalues of geometric factors between 0° and 90°. The interpolationresults for the geometric factors at different deviation angles,G_(osθ), as a function of k_(H)/k_(V) are presented in Table 7 and FIG.13. TABLE 7 Geometric factor values (G_(osθ)) at different deviationangles K_(H)/K_(V) 0 22.5 45.0 67.5 90 0.01 3.75 4.96 6.18 7.40 8.62 0.13.64 4.20 4.76 5.31 5.87 1 4.08 4.08 4.08 4.08 4.08 10 5.42 4.75 4.073.40 2.73 100 8.33 6.71 5.09 3.47 1.86 1000 14.18 10.95 7.72 4.49 1.2610000 25.96 19.68 13.41 7.14 0.86 100000 49.64 37.38 25.11 12.85 0.591000000 97.09 72.92 48.74 24.57 0.40

We may also use the geometric skin factor, s_(pθ), to account for thenon-spherical flow. Similarly, the values of the geometric skin factorcan be derived using an interpolation. The derived values of s_(pθ) arepresented in Table 8 and FIG. 14. TABLE 8 Geometric skin factor values(s_(pθ)) at different deviation angles K_(H)/K_(V) 0 22.5 45.0 67.5 900.01 2.35 1.88 1.41 0.93 0.46 0.1 2.45 2.12 1.80 1.47 1.14 1 2.08 2.082.08 2.08 2.08 10 1.32 1.89 2.46 3.04 3.61 100 0.51 1.82 3.14 4.45 5.771000 −0.11 2.15 4.41 6.67 8.93 10000 −0.52 3.01 6.53 10.06 13.58 100000−0.75 4.54 9.83 15.12 20.40 1000000 −0.87 6.95 14.77 22.59 30.42The values of geometric factor and the geometric skin factor in Tables13 and 14 are for positions 2 or 4 of the probe, i.e., φ=90° or 270°.Values for other positions will be different.

To determine correct permeabilities, two tests at Positions 1 and 2 areconducted. For the test at Position 1, the relationship between thepressure drop and flow rate is the same as that in a vertical well.Using the geometric factor G_(os) and spherical permeability k_(s), therelationship is given by eqn. (20) and reproduced here: $\begin{matrix}{{{p_{i} - p_{p}} = \frac{q\quad\mu}{G_{os}k_{s}r_{p}}},} & (45)\end{matrix}$where G_(os) is the geometric factor when the pressure drop vs. flowrate relationship is formulated using spherical permeability, k_(s).

For the test at Position 2, the relationship between the pressure dropand flow rate is described using Eqn. 46 following the notation used ina vertical well with the geometric factor G_(os) replaced by the value(G_(osθ)): $\begin{matrix}{{p_{i} - p_{p}} = {\frac{q\quad\mu}{G_{{os}\quad\theta}k_{s}r_{p}}.}} & (46)\end{matrix}$Either Eqn. (45) or (46) can be used to obtain the sphericalpermeability. However, the geometric factors in these equations arestrong functions of k_(H)/k_(V). Before analyzing the test data,k_(H)/k_(V) is unknown. Therefore, the spherical permeability cannot bedirectly obtained. However, for a particular test with the measured qand p_(p), and the fixed μ, r_(p), the product G_(os)k_(s) orG_(osθ)k_(s) is a determined quantity. In other words, when the twoprotests are conducted at the same measured depth, we can obtain twoquantities: $\begin{matrix}{{{K_{1} \equiv {G_{os}k_{s}}} = \frac{q_{1}\mu}{r_{p}\left( {p_{i} - p_{p\quad 1}} \right)}},{and}} & (47) \\{{{K_{2} \equiv {G_{{os}\quad\theta}k_{s}}} = \frac{q_{2}\mu}{r_{p}\left( {p_{i} - p_{p\quad 2}} \right)}},} & (48)\end{matrix}$where the subscripts 1 and 2 represent the test at Positions 1 and 2,respectively. Both K₁ and K₂ are functions of permeability anisotropyrepresented by k_(H)/k_(V). Now we define another quantity K using thesetwo quantities: $\begin{matrix}{{K_{\theta} \equiv \frac{K_{1}}{K_{2}}} = {\frac{G_{{os}\quad\theta}}{G_{os}}.}} & (49)\end{matrix}$

Using the values of G_(osθ) in Table 7 and G_(os) from Table 1, thevalues of K_(θ) are obtained as shown in Table 9 and FIG. 15 as afunction of k_(H)/k_(V) and 0, with φ=90° or 270°. Note that the K_(θ)values here are for φ=90° or 270°. The K_(θ) values at other φ must begenerated using the values of G_(osθ) at other φ. TABLE 9 K_(θ) valuesfor different k_(H)/k_(V) at different deviation angles (φ = 90° or270°) K_(H)/K_(V) 0 22.5 45.0 67.5 90 0.01 1.0000 1.3250 1.6500 1.97502.3000 0.1 1.0000 1.1532 1.3064 1.4596 1.6128 1 1.0000 1.0000 1.00001.0000 1.0000 10 1.0000 0.8757 0.7514 0.6271 0.5028 100 1.0000 0.80580.6115 0.4173 0.2230 1000 1.0000 0.7723 0.5446 0.3169 0.0892 100001.0000 0.7583 0.5166 0.2749 0.0332 100000 1.0000 0.7530 0.5059 0.25890.0118 1000000 1.0000 0.7510 0.5021 0.2531 0.0041

For the two pretests conducted at the same measured depth, the K valuecan be calculated using K₁ and K₂ from Eqns. 47 and 48, respectively.Then the k_(H)/k_(V) at the measured depth can be obtained from thelook-up table 9 or FIG. 15 using the calculated K value and the knownvalue of deviation angle. Once we know k_(H)/k_(V), the correct valuesof G_(os) and G_(osθ) can be determined. Thus the correct sphericalpermeability can be determined from either Eqns. 47 and 48. Thehorizontal and vertical permeabilities are readily determined from thespherical permeability and k_(H)/k_(V): $\begin{matrix}{{k_{H} = {K_{s}\left( \frac{k_{H}}{k_{V}} \right)}^{1/3}}{and}} & (50) \\{k_{V} = {\frac{k_{H}}{\left( {k_{H}/k_{V}} \right)}.}} & (51)\end{matrix}$

The above formulas are presented in terms of drawdown equation based onthe assumptions of a constant rate and steady state flow. The steadystate flow condition cannot be satisfied in a low permeabilityformation, or a long test time is needed. A constant drawdown rate maynot reachable in practice because the tool needs time for accelerationand deceleration. The storage effect also makes it difficult to reach aconstant rate. To overcome these inabilities, the combination methoddescribed above using buildup and drawdown should be used to calculateK₁ and K₂.

The embodiment of the invention described immediately above teaches amethod to determine correct spherical permeability, horizontal andvertical permeabilities by conducting two probe tests in two differentdirections in a deviated well of arbitrary deviation. Earlier, anembodiment in which the permeabilities were determined by making twomeasurements in a substantially horizontal wellbore was discussed. Inyet another embodiment of the invention, the determination of thepermeabilities may be made by conducting only one test at one position.Where one test is conducted, then the test should have a drawdown periodfollowed by a buildup period. If the test is conducted at Position 1,the analysis procedures are the same as those described above using thedrawdown and buildup measurements. If the test is conducted at Position2, the analysis procedures are similar, except that the geometric factorvalues should be replaced by the values of G_(osθ) listed in Table 7corresponding to the well deviation angle, or the geometric skin factorvalues should be replaced by the values of s_(pθ) listed in Table 8.

When the probe is set at Position 1 or Position 3, the values of thegeometric factor or geometric skin factor are unchanged with the welldeviation angle. This leads to an important practical application information testing. In an actual deviated well, the deviation angles aredifferent at different measured depths. We know that the values ofgeometric factor or geometric skin factors are a function of deviationangle. The linear interpolation discussed above may only give anapproximate value of the geometric factor and geometric skin factors. Intests conducted at different measured depths by setting probe atdifferent positions (different angles φ), the analysis results aresubject to this approximation. For tests conducted with probes set atPosition 1 or Position 3, the analysis results are certain, and thecomparison of analysis results can be simplified by avoiding the effectof deviation angle.

The invention has been described in terms of measurements made usinglogging tools conveyed on a wireline in a borehole. As noted above, Themethod can also be used on data obtained usingmeasurement-while-drilling sensors on a bottomhole assembly (BHA)conveyed by a drilling tubular. Such a device is described, for example,in U.S. Pat. No. 6,640,908 to Jones et al., and in U.S. Pat. No.6,672,386 to Krueger et al., having the same assignee as the presentinvention and the contents of which are fully incorporated herein byreference. The method disclosed in Krueger comprises conveying a toolinto a borehole, where the borehole traverses a subterranean formationcontaining formation fluid under pressure. A probe is extended from thetool to the formation establishing hydraulic communication between theformation and a volume of a chamber in the tool. Fluid is withdrawn fromthe formation by increasing the volume of the chamber in the tool with avolume control device. Data sets are measured of the pressure of thefluid and the volume of the chamber as a function of time.

The embodiments of the invention that require making measurements on twodifferent walls of a substantially horizontal borehole are readilyaccomplished in a MWD implementation. If the tests are performed afterthe well has been drilled, several options are available. One is toconvey the pressure tester on coiled tubing. Alternatively, a downholetraction device such as that disclosed in U.S. Pat. No. 6,062,315 toReinhardt, having the same assignee as the present invention and thecontents of which are fully incorporated herein by reference, may beused to convey the pressure tester into the borehole. A traction devicemay also be used to withdraw the pressure tester from the borehole, or,alternatively, the withdrawal may be done using a wireline.

The processing of the measurements made by the probe in wirelineapplications may be done by the surface processor 21 or may be done by adownhole processor (not shown). For MWD applications, the processing maybe done by a downhole processor that is part of the BHA. This downholeprocessing reduces the amount of data that has to be telemetered.Alternatively, some or part of the data may be telemetered to thesurface. In yet another alternative, the pressure and flow measurementsmay be stored on a suitable memory device downhole and processed whenthe drillstring is tripped out of the borehole.

The operation of the probe may be controlled by the downhole processorand/or the surface processor. The term processor as used in thisapplication includes such devices as Field Programmable Gate Arrays(FPGAs). Implicit in the control and processing of the data is the useof a computer program implemented on a suitable machine readable mediumthat enables the processor to perform the control and processing. Themachine readable medium may include ROMs, EPROMs, EAROMs, Flash Memoriesand Optical disks.

While the foregoing disclosure is directed to the specific embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all such variations within thescope and spirit of the appended claims be embraced by the foregoingdisclosure.

1. A method of estimating a permeability of an earth formation, theformation containing a formation fluid, the method comprising: (a)making transient pressure measurements with a probe in a borehole in theearth formation, the probe having an aperture that is one of (A)substantially circular aperture, and (B) non-elliptical, the aperturebeing in hydraulic communication with the earth formation; (b) obtaininga first and a second estimate of permeability from the pressuremeasurements; and (c) determining from the first and second estimates ofpermeability at least one of (A) a horizontal permeability of theformation, and (B) a vertical permeability of the formation.
 2. Themethod of claim 1 wherein the pressure measurements further comprise:(i) a drawdown involving withdrawal of fluid from said earth formationby increasing a volume of a chamber in hydraulic communication with saidprobe and measuring said volume; and (ii) stopping the withdrawal of thefluid at a defined time; wherein obtaining the first estimate ofpermeability further comprises analyzing a first set of pressuremeasurements made after the defined time and wherein obtaining thesecond estimate of permeability further comprises analyzing measurementsmade before and after the defined time
 3. The method of claim 1 furthercomprising determining at least one of (I) a ratio of a horizontalpermeability and a vertical permeability of the formation, and (II) aspherical permeability of the formation.
 4. The method of claim 1wherein step (c) further comprises using at least one of (i) a geometricfactor relating to anisotropy, and (ii) a geometric skin factor relatingto non-spherical fluid flow.
 5. The method of claim 2 further comprisingobtaining a geometric skin factor by matching pressure measurements madebefore and after said defined
 6. The method of claim 1 whereindetermining said first permeability further comprises using a relationof the form:${p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{{\phi\mu}\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}$c_(t) is a total formation compressibility; k_(s) is a sphericalpermeability; p(t) represents a measured pressure in the tool; p_(i) isthe initial formation pressure; q is a volumetric flow rate; r_(p) is atrue probe radius; t is a time after said defined time; μ is a viscosityof fluid; and φ is a formation porosity, fraction.
 7. The method ofclaim 1 further comprising conveying the probe into the borehole on oneof (i) a wireline, (ii) a drilling tubular, and (iii) a slickline. 8.The method of claim 1 wherein the borehole is substantially vertical andthe first and second estimates of permeability are obtained with theprobe in substantially the same orientation in the borehole.
 9. Themethod of claim 1 wherein the borehole comprises a deviated borehole.10. The method of claim 9 wherein the probe aperture lies substantiallyin a vertical plane.
 11. An apparatus for evaluating an earth formation,the formation containing a formation fluid, the apparatus comprising:(a) a probe conveyed in the borehole, the probe being in hydrauliccommunication with the earth formation and having an aperture that isone of (A) substantially circular, and (B) non-elliptical; (b) apressure sensing device which makes transient pressure measurements insaid probe; and (c) a processor which: (A) estimates a firstpermeability and a second permeability from the pressure measurements,and (B) determines from the first and second permeability at least oneof (I) a horizontal permeability of the formation, and (II) a verticalvertical permeability of the formation.
 12. The apparatus of claim 11wherein the pressure measurements further comprise: (i) a drawdowninvolving withdrawal of fluid from the earth formation by increasing avolume of a chamber in hydraulic communication with the probe andmeasuring said volume; and (ii) stopping the withdrawal of the fluid ata defined time; wherein the processor obtains the first estimate ofpermeability at least in part by analyzing a first set of pressuremeasurements made after the defined time and wherein the processorobtains the second estimate of permeability at least in part byanalyzing measurements made before and after the defined time
 13. Theapparatus of claim 11 wherein the processor further determines at leastone of (I) a ratio of a horizontal permeability and a verticalpermeability of the formation, and (II) a spherical permeability of theformation.
 14. The apparatus of claim 11 wherein the processor performsstep (B) by using at least one of (i) a geometric factor relating toanisotropy, and (ii) a geometric skin factor relating to non-sphericalfluid flow.
 15. The apparatus of claim 12 wherein the processor furtherestimates a geometric skin factor by matching pressure measurements madebefore and after said defined time.
 16. The apparatus of claim 11wherein said processor determines the first permeability further using arelation of the form below:${p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{{\phi\mu}\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}$c_(t) is a total formation compressibility; k_(s) is a sphericalpermeability; p(t) represents a measured pressure in the tool; p_(i) isthe initial formation pressure; q is a volumetric flow rate; r_(p) is atrue probe radius; t is a time after said defined time; μ is a viscosityof fluid; and φ is a formation porosity, fraction.
 17. The apparatus ofclaim 11 further comprising a conveyance device which conveys the probeinto the borehole, the conveyance device selected (i) a wireline, and(ii) a drilling tubular.
 18. The apparatus of claim 11 wherein theborehole is substantially vertical and the processor estimates the firstand second permeability from measurements made by the probe insubstantially the same orientation in the borehole.
 19. The apparatus ofclaim 11 wherein the borehole comprises a deviated borehole.
 20. Theapparatus of claim 19 wherein the probe aperture lies substantially in avertical plane.
 21. A computer readable medium for use with an apparatusfor evaluating an earth formation, the formation containing a formationfluid, the apparatus comprising: (a) a probe conveyed in the borehole,the probe being in hydraulic communication with the earth formation andhaving an aperture that is one of (A) substantially circular, and (B)non-elliptical; and (b) a pressure sensing device which makes transientpressure measurements in said probe; the medium comprising instructionswhich enable a processor to (c) estimate a first permeability and asecond permeability from the pressure measurements, and (d) determinefrom the first and second permeability at least one of (I) a horizontalpermeability of the formation, and (II) a vertical permeability of theformation.
 22. The medium of claim 21 further comprising at least oneof: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a Flash Memory, and(v) an optical disk.